The design requirements for this clamping fixture stem from the need to accommodate small degrees of misalignment in brittle specimens which are to be subjected to fully reversed axial or torsional loads. Important information of the mechanical properties of a material can be obtained from uniaxial tests in which the stress or strain can be cycled freely between tension and compression. For example, damping and deformation mechanisms, modulus variations and cyclic stress-induced damage can be studied. The capability of fully reversing stress allows the examination of material behavior under loading paths which cannot be attained by other experimental methods.
Despite the usefulness of the reversed direct stress techniques, its application has generally be been limited to easily machined manufactured materials. The application of this technique to more brittle materials and specimens of geologic materials, such as ice, has been hampered by the difficulties associated with rigidly fixing the specimen in a loading device. It is difficult to mount a material having relatively low fracture strength in a stress testing device. Misalignment or geometric imprecision common in specimens of naturally-occurring materials must be accommodated by the test fixtures in order to avoid damage or complete mechanical failure of the test piece will occur while it is being attached to the testing machine.
Hydraulically-actuated systems for gripping slightly misaligned specimens of brittle materials exist. These systems generally employ a ball seat to correct for specimen misalignment, but are designed for a very narrow range of specimen geometries and are generally very expensive. The current state-of-the-art systems have in common the feature that the ball seat used for correction of the alignment error is located at some distance from the source of the error, not at the specimen itself. When even the lightest misalignment is corrected for in this manner, the stress state in the test piece deviates from the intended pure stress field. This condition can seriously affect the validity of the test results.
In view of the above, it can be seen that there is a need for a device which allows for rapid and precise gripping of a test specimen in a stress testing device which avoids the introduction of unintended bending moments or axial loads to the specimen.